1. Technical Field
The present invention relates to an image processing device, an image output device, a terminal device, and an image forming system and a computer readable medium storing a program thereof.
2. Related Art
Trapping is a processing method which, when printing two adjacent images, makes either of the adjacent images a bit larger so that the images are partially overlapped when printed. Specifically, as is illustrated in FIG. 1, by spreading a background image (choking a white blank portion), this method makes an overlap of a black character image onto the background image. If an image including a black character object and its background manipulated by such trapping is printed, white edges will not appear even if, for example, printing misregistration occurs, as is illustrated in FIG. 1.
Overprinting is a processing method in which no white blank area is provided in the background, as is illustrated in FIG. 2. This overprinting also produces a print result in which white edges do not appear even if, for example, printing misregistration occurs, as is illustrated in FIG. 2.
However, when color conversion such as CMYK-CMYK conversion is performed on an image manipulated by the above trapping, overprinting, and the like, such a phenomenon as will be described below occurs.
For example, consider a case where color conversion is performed on an image in which a black character image (K) is overlaid on a background image whose color is composed of a combination of colors (C1, M1, Y1), as is illustrated in FIG. 3A; in this state, pixel values in a region where the black character image is superposed on the background image are C1, M1, Y1, K and pixel values in a region where only the background image exists are C1, M1, Y1, 0.
Through color conversion for the pixels in the region where only the background image exists, the pixel values of C1, M1, Y1, 0 are converted to C2, M2, Y2, 0, as illustrated in FIG. 4A. Through color conversion for the pixels in the region where the black character image is superposed on the background image, the pixel values of C1, M1, Y1, K are converted to C3, M3, Y3, K, as illustrated in FIG. 4B. Color conversion from CMYK image data to CMYK image data is performed such that each set of all combinations of the values of the four colors of CMYK is replaced by the corresponding set among the combinations of replacement CMYK values which are predefined. Thus, the sets having even the same CMY values, but plus a different value of K, are converted to the sets in which different CMY values are assigned.
Consequently, as illustrated in FIG. 3B, some portions of the background image are converted to C2, M2, Y2 and other portion thereof is converted to C3, M3, Y3, depending on whether the black character exists on the background, and these portions become to have different hues in the color-converted image.